Contemporary lubricants such as engine oils use a wide variety of additives for enhancing the performance of the lubricants over a wide range of operating conditions. For example, additives are used to improve the antiwear performance of lubricants, to reduce friction characteristics, to increase oxidation stability and the like.
Traditionally, the principal antiwear additive for engine oils has been zinc dialkyldithiophosphate (ZDDP); and in order for a lubricating composition to meet industry standard tests and many original equipment manufacturer's specified tests for antiwear, the amount of ZDDP used in the oil is at a concentration that provides a phosphorous content in the oil of 0.08 wt % or higher. One problem arising from the use of current levels of ZDDP is the reduction of the efficiency of the catalyst converter now universally employed with gasoline powered vehicles. As a consequence, there is a new focus on reducing the levels of phosphorous in lubricating oils, especially engine oils.
Reducing the level of ZDDP in a lubricating composition, however, results in a significant reduction in the antiwear performance of the lubricating composition. For example, reducing the level of ZDDP by one half from the typical 0.08 wt % phosphorus level can result in as much as a seven-fold decrease in the antiwear performance of the lubricating composition.